1. Field of the Invention
The present invention relates to an optical encoder for detecting movement information with high accuracy.
2. Related Background Art
The conventionally known methods for detecting the position or speed of a moving object are roughly classified in methods with a magnetic encoder and methods with an optical encoder. Optical encoders are usually comprised of a light-projecting section, a light-receiving section, and a scale and the scale is normally made of a thin SUS material by precise press blanking or by etching.
In recent years, however, suggestions have been made on the optical encoders using the scale of a transparent material provided with grooves of V-shaped cross section, for example, as described in Japanese Patent Application Laid-Open No. 11-23324 etc., and they are used in printers, copying machines, and so on.
FIG. 1 is a perspective view of an optical system in a self-emitting optical encoder of a conventional example and FIG. 2 is a cross-sectional view thereof. The optical encoder is provided with a light-irradiating device 3 comprised of a light source 1 such as an LED or a semiconductor laser for emitting coherent light, for example, of the wavelength of 632.8 nm, and a lens system 2 consisting of a spherical lens or an aspherical lens; an optical scale 4 with a grating having the phase difference detecting function and amplitude diffraction grating function; a concave mirror 5 having a curved surface matching with the Fourier transform surface of the grating and having the optical axis O1 decentered by a center difference xcex94 relative to the optical axis O of a central beam of incident light; and a light-receiving device 6 consisting of light-receiving elements 6a, 6b, 6c being three photodetectors. The output of the light-receiving device 6 is connected to a signal processing unit 7 having a pulse-counting circuit and a rotational direction determining circuit, and the light-irradiating device 3 and light-receiving device 6 are held in a fixed state in a housing 8. The optical scale 4 is attached to part of a rotating body not illustrated and is under rotation in the direction of an arrow D about the rotational axis O2 together with the rotating body.
FIG. 3 is a plan view of the optical scale in which the grating of the optical scale 4 is formed so that two slopes I1, I2 forming a V-groove, and one flat F appear alternately at a predetermined pitch P and are formed continuously in radial directions, as illustrated in FIGS. 4A, 4B. The width of the V-groove is P/2, and each of the two slopes I1, I2 forming the V-groove has the width of P/4 and is inclined at an angle not less than the critical angle, for example, at the angle xcex8=45xc2x0, relative to the flat F.
The grating has a first region 4a of the shape illustrated in FIG. 4A radially inside and a second region 4b of the shape illustrated in FIG. 4B radially outside. Each of FIG. 4A and FIG. 4B includes a front view and a cross-sectional view of the corresponding region. Since the scale grooves are radially continuous, the number N1 of V-grooves in the first region 4a is equal to the number N2 of V-grooves in the second region 4b (N1=N2). A ratio (R2/R1) of the distance R2 from the rotation center O2 of the optical scale 4 to the second region 4b, to the distance R1 similarly to the first region 4a is equal to a ratio (P2/P1) of the scale pitch P2 of the second region 4b to the scale pitch P1 of the first region 4a (i.e., R2/R1=P2/P1).
The light from the light source 1 being one element of the light-emitting device 3 is condensed by the lens system 2 onto the optical scale 4. The light incident to the first region 4a of the optical scale 4 is diffracted by the grating and the nth-order diffracted light (0-order and xc2x11-order diffracted light) is condensed at or near the pupil position of the concave mirror 5.
The concave mirror 5 reflects these three diffracted light beams thus condensed to form an interference pattern image based on these three diffracted beams in the second region 4b on the surface of the optical scale 4. At this time, with movement of the optical scale 4 in the rotation direction D, the thus formed image moves in the direction opposite to the rotation direction D. Namely, the interference pattern image is displaced relative to the grating by double the movement of the optical scale 4. This enables acquisition of rotation information in the resolution of double the grating formed in the optical scale 4.
Beams based on the phase relation between the interference pattern image formed near the second region 4b of the optical scale 4 and the V-grooves of the grating are geometrically refracted by the second region 4b, three beams emerging from the second region 4b are received by the three light-receiving elements 6a, 6b, 6c of the light-receiving device 6, respectively, and signals from this light-receiving device 6 are processed by the signal processing unit 7 to obtain the rotation information.
FIG. 5A shows the convergent light incident onto the grating of the first region 4a of the optical scale 4, and beams arriving at the flats F of the grating among the light travel through the flats F toward the concave mirror 5 to be focused on the surface thereof. Since the slope angle of the slopes I1 is set over the critical angle, a beam arriving at each slope I1 forming the V-groove is totally reflected toward the other slope I2 together forming a V-groove and then is totally reflected again by the slope I2.
In this manner the beams finally arriving at the slopes I1 of the grating are reflected back opposite to the incident direction without entering the inside of the optical scale 4. Likewise, the beams arriving at the other slopes I2 are also totally reflected twice back opposite to the incident direction. Therefore, the beams arriving at the two slopes I1, I2 are not transmitted but reflected by the optical scale 4, whereas only the beams arriving at the flats F travel through the optical scale 4, in the first region 4a. 
In the first region 4a the V-grooved grating has the optical action similar to the transmissive amplitude diffraction grating. Namely, the light is diffracted by the grating of the first region 4a to generate beams of 0-order, xc2x11-order, xc2x12-order, . . . diffracted light by the action of the grating, and the beams are condensed on the surface of the concave mirror 5. The diffracted light thus condensed is reflected by the concave mirror 5 to enter the second region 4b of the optical scale 4, as illustrated in FIG. 5B, thereby forming an image of radial grooves on the surface of the optical scale 4. Since the first region 4a and the second region 4b are radially different regions (which may overlap with each other in part) of the radial grating on the surface of the optical scale 4, the grating pitches of the first region 4a and the second region 4b are different from each other, and the inside and outside pitches of the optical scale 4 are also different even in the irradiation area of the second region 4b. 
In this prior art example, therefore, the grating of the first region 4a is enlargingly projected onto the second region 4b so that a reversed image thereof may be formed at the same pitch as that of the radial grating of the optical scale 4. For this purpose, the concave mirror 5 is designed to have a desired radius R of curvature and be decentered from the optical axis O of the incident light and the deviation xcex94 of the concave mirror 5 from the optical axis O of incidence is set so as to make the enlargement projection magnification optimum. In this way the pitches of the radial grating are matched in part for formation of the grating image of the first region 4a on the surface of the second region 4b by the concave mirror 5, thereby obtaining detection signals with good S/N ratios.
The beams incident to the flats F in the second region 4b travel straight relative to the slopes I1, I2, as illustrated in FIG. 5C, to reach the center light-receiving element 6b of the light-receiving device 6. Since the beams arriving at the two slopes I1, I2 forming the V-grooves are incident at the angle of incidence of 45xc2x0 to each surface, the beams are largely refracted into directions different from each other, to reach the light-receiving elements 6a, 6c at the both ends of the light-receiving device 6.
In the second region 4b the beams thus travel in the three separate directions because of the totally three types of surfaces along the different slope directions, the two slopes I1, I2 inclined in the different directions to the incident light, and the flat F between V-grooves, and then they reach the respective light-receiving elements 6a, 6b, 6c provided at their respective positions corresponding to the surfaces. Namely, the beams based on the phase relation between the grating of the second region 4b and the interference pattern image formed on the surface thereof are deflected into the three directions to be focused on the respective light-receiving elements 6a, 6b, 6c, and thus the grating of V-grooves functions as a lightwave wavefront splitting element in the second region 4b. 
With rotation of the optical scale 4, there will occur variation in amounts of light detected by the respective light-receiving elements 6a, 6b, 6c. A light-amount balance among the beams incident to the respective light-receiving elements 6a, 6b, 6c varies according to relative displacement between the position of the grating and the position of the interference pattern image. As a result, in the case of counterclockwise rotation of the optical scale 4, the light-amount variation as illustrated in FIG. 6 appears with rotation of the optical scale 4. In this figure, the horizontal axis represents rotation amounts of the optical scale 4, the vertical axis represents amounts of received light, and signals a, b, c correspond to outputs of the respective light-receiving elements 6a, 6b, 6c. In the case of clockwise rotation of the optical scale 4 on the other hand, the signal a indicates the output of the light-receiving element 6b, the signal b the output of the light-receiving element 6a, and the signal c the output of the light-receiving element 6c. Pulse signals are generated based on these signals and are processed to yield the rotation information such as the angle or amount of rotation of the optical scale 4, or rotating speed, rotating acceleration, etc. thereof. FIG. 6 shows the theoretical light-amount variation obtained when the contrast of the interference pattern image formed on the second region 4b is very high and almost ideal.
FIG. 7 is a perspective view of a second conventional example, in which the optical encoder is provided with a light source 11 such as the LED or the semiconductor laser, a lens system 12 consisting of a spherical lens or an aspherical lens for converting divergent light from the light source 11 into parallel light, a scale 13 having the amplitude grating function and being driven to rotate, a fixed scale 14 consisting of two grating portions 14a, 14b having the same pitch as that of the optical scale 13, and a light-receiving device 15 having two light-receiving elements 15a, 15b set with a phase shift of a quarter pitch.
The substrate of the optical scale 13 is made of a transparent optical material and a grating portion 13a, in which a plurality of opaque portions are radially formed in fixed periods, is provided on the surface of the transparent substrate. It is also possible to employ such structure that the substrate of an opaque material is provided with the grating portion having a plurality of radially long holes in fixed periods, on the other hand. The optical scale 13 is attached to part of the rotating body not illustrated and is rotated in the direction of arrow D about the rotation axis O3 together with the rotating body.
The light emitted from the light source 11 is converted into parallel light by the lens system 12 to travel through the optical scale 13 and the fixed scale 14. The light passing through the grating portion 14a of the fixed scale 14 is received by the light-receiving element 15a of the light-receiving device 15, while the light passing through the grating portion 14b by the light-receiving element 15b. 
Since the optical scale 13 is rotating about the rotation axis O3, the light-receiving device 15 receives a maximum amount of light from the light source 11 when the phase of the grating portion 13a of the optical scale 13 becomes coincident with the phase of the grating portion 14a or 14b of the fixed scale 14. In contrast, the light-receiving device 15 receives a minimum amount of light when the phases are opposite. Therefore, amounts of light detected by the light-receiving elements 15a, 15b vary with rotation of the optical scale 13. Namely, the light-amount balance between the beams incident to the respective light-receiving elements 15a, 15b varies according to the relative change between the position of the grating portion 13a and the image position.
FIG. 8 is a graph to show the light amount variation with rotation of the optical scale 13, in which the horizontal axis represents rotation amounts of the optical scale 13 and the vertical axis amounts of received light. This FIG. 8 shows the state of theoretical light amount variation obtained when the contrast is very high and almost ideal. With counterclockwise rotation of the optical scale 13, the light-receiving elements 15a, 15b output the light amount variations indicated by signals a, b, respectively. With clockwise rotation of the optical scale 13 on the other hand, the light-receiving element 15a provides the output of the light amount variation of signal b, and the light-receiving element 15b the output of the light amount variation of signal a. Pulse signals are generated based on these signals and are used to detect the rotation information such as the angle or amount of rotation of the optical scale 13, or the rotating speed, the rotating acceleration, etc. thereof.
For accomplishing improvement in the conventional examples as described above, a first object of the present invention is to provide an optical encoder capable of always generating stable pulses without variation in the width and phase even if there occurs variation in amounts of light.
A second object of the present invention is to provide an optical encoder capable of detecting signals with good contrast against scales of all diameters by use of a common detection head.
A third object of the present invention is to provide an optical encoder capable of detecting displacement information with good contrast in compact structure, particularly, in terms of the axial height.